1. Field of the Invention
This invention relates to time delay circuits for optical devices and more particularly to a method and apparatus to overcome problems associated with the drift of the bias voltages in optical switches and allow the calibration of the circuit during use so that the system may continue running without returning to the laboratory for re-calibration or without having excessive components which add to the weight, space and power consumption of the system.
2. Description of the Prior Art
Fiber optic systems have become common and find many uses as, for example, in communications, signal processing and detection. In such systems, light sources are employed which often are modulated (digital, amplitude, frequency or phase) to delays are utilized in many systems such as for steering the direction of a radar or microwave beam in a Phased Array Antenna for both transmitting and receiving modes, for optical path matching in a distributed optical sensing system using a light source with either low coherence (e.g. LED) or high coherence (e.g. laser), or for sampling the airspeed at a particular distance window ahead of a craft using a laser Doppler velocimeter (air data sensor). At the present time, variable optical delay devices are available in the form of a plurality of optical lines each of different length and one of the lines is used for each delay time desired. When a large number of delay times is desired, such systems become impractical. For example, if it is desired to provide 32 different delay times, the prior art would have to utilize 32 different lines in some sort of package. This also requires some sort of switching arrangement to choose the proper line and may require 32 identical light sources, such as laser diodes, all modulated on the same carrier frequency. It will be appreciated that the cost and complexity of such systems becomes prohibitive. While theoretically possible to use a single source and a switch that could connect it to one of the 32 lines, at this stage of the art such switches would have to be mechanical (and thus slow) or tiers of 2.times.2 integrated optic switches as described in--Chapter 6 pages 153-155 of a book entitled "Antenna Design With Fiber Optics" by A. Kumar and published in 1996 by Artech House of Boston and London (Library of Congress Catalog Card Number 95-49990). Unfortunately, optical delay systems using such switches would involve a great deal of light loss.
A solution to this problem is described and claimed in the above mentioned Ang et al application by providing a time delay unit with a switch and two paths for a light beam, which, for convenience, will be called an upper and a lower path, the upper path being longer than the lower path by an amount sufficient to produce a predetermined delay of the beam in the upper path with respect to the beam in the lower path. The switch, when properly biased, will cause the beam to stay in one path through put or cross to the other path (cross circuit) or, in other words, to stay in the upper or lower path or to change paths from lower to upper or vice versa. In order to provide a number of different time delays, more of the delay units can be activated to effect various combinations of delay and produce a desired overall delay. For example, if 64 different time delays were desired, six time delay units may be used in series and the basic time delay T can be transformed to a delay of T.times.(2.sup.6) different values. The minimum delay being 0.times.T and the longest delay being 63.times.T. For manufacturing simplicity, each time delay unit may be made up of one or more of the basic units. Thus, in binary fashion, the first unit may be one basic unit, the second unit may be two basic units, the third may be four basic units etc. The advantages of using optical carrier is making possible switching ease, high speed, providing a large dynamic range of delays (i.e. picoseconds to milliseconds) with little or no dispersion due to different modulation frequencies (i.e. time delay at 1 Ghz will be the same as the time delay at 100 Ghz.), modulated signal microwave ripple and deviation from linear phase independent of time delay, and savings in space and weight. Of course, it will be realized that a binary circuit is not required and other arrangements of different length fibers may be use to obtain desired overall delay times.
While providing a great advantage over presently available delay devices, the bias voltages applied to the switches of the above described Ang et al system may drift over time with the result that the light does not go solely to the correct path (a significant portion of the light could leak to the wrong path if the switches are not properly adjusted) and the performance of the system is degraded. While previous systems could be returned to the laboratory for recalibration, this is highly undesirable. With available integrated optical switches the drift is so bad that the desired high performance can not be realized without some sort of in situ recalibration. It might be possible to calibrate the switches in situ by providing individual monitors for each output port of each light switch to correct any drift occurring in that switch but having a separate monitor for each of the switches would be very much more costly in terms of size, weight and power consumption than is desirable.